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Tons of uranium residues have to be treated in order to obtain a few decigrammes of pure salts of radium. Radium is characterised by a special spectrum, and its atomic weight, as determined by Madame Curie, is 225; it is consequently the higher homologue of barium in one of the groups of Mendeléef.
Perhaps the reader may feel inclined to say with the Irishman that all this is "as dry as ditch-water," but he will see before long that it has a good deal to do with ourselves. For the present what I want him to realize by a few examples is the mathematical accuracy of Law. The value of these examples lies in their illustration of the fact that the Law can always be trusted to lead us on to further knowledge. We see it working under known conditions, and relying on its unchangeableness, we can then logically infer what it will do under other hypothetical conditions, and in this way many important discoveries have been made. For instance it was in this way that Mendeléef, the Russian chemist, assumed the existence of three then unknown chemical elements, now called Scandium, Gallium and Germanium. There was a gap in the orderly sequence of the chemical elements, and relying on the old maxim "Natura nihil facit per saltum" Nature nowhere leaves a gap to jump over he argued that if such elements did not exist they ought to, and so he calculated what these elements ought to be like, giving their atomic weight, chemical affinities, and the like; and when they were discovered many years later they were found to answer exactly to his description. He prophesied, not by guesswork, but by knowledge of the Law; and in much the same way radium was discovered by Professor and Madame Curie. In like manner Hertz was led to the discovery of the electro-magnetic waves. The celebrated mathematician Clerk-Maxwell had calculated all particulars of these waves twenty-five years before Hertz, on the basis of these calculations, worked out his discovery. Again, Neptune, the outermost known planet of our system was discovered by the astronomer Galle in consequence of calculations made by Leverrier. Certain variations in the movements of the planets were mathematically unaccountable except on the hypothesis that some more remote planet existed. Astronomers had faith in mathematics and the hypothetical planet was found to be a reality. Instances of this kind might be multiplied, but as the French say "
That the atoms of all the elements must have a common unit of composition, that they behave as if composed of ultimate particles that may be regarded as duplicates of one another, has long been regarded as an inevitable conclusion from the Periodic Law of Mendeleef.
Professor Mendeleef, in Russia, resolved to engage a balloon, and by rising above the cloud barrier, should there be one, to have the eclipse all to himself.
We may believe with Professor Ostwald and his followers that the molecules of sugar merely glide everywhere between the molecules of water, without chemical action; or, on the other hand, dismissing this mechanical explanation, we may say with Mendeleef that the process of solution is the most active of chemical phenomena, involving that incessant interplay of atoms known as dissociation.
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